1. Field of the Invention
The present invention relates to single acting, variable displacement fluid pressure vane pumps and motors for aircraft use, component parts thereof and to a method for balancing fluid pressures.
Over the years, the standard of the commercial aviation gas turbine industry for main engine fuel pumps has been a single element, pressure-loaded, involute gear stage charged with a centrifugal boost stage. Such gear pumps are simple and extremely durable, although heavy and inefficient. However, such gear pumps are fixed displacement pumps which deliver uniform amounts of fluid, such as fuel, under all operating conditions. Certain operating conditions require different volumes of liquid, and it is desirable and/or necessary to vary the liquid supply, by means such as bypass systems which can cause overheating of the fuel or hydraulic fluid and which require heat transfer cooling components that add to the cost and the weight of the system.
2. State of the Art
Vane pumps and systems have been developed in order to overcome some of the deficiencies of gear pumps, and reference is made to the following U.S. Pat. Nos. for their disclosures of several such pumps and systems: 4,247,263; 4,354,809; 4,529,361 and 4,711,619. Reference is also made to copending commonly-owned application U.S. Ser. No. 08/114253, filed Aug. 30, 1993, the disclosure of which is hereby incorporated herein.
Vane pumps comprise a rotor element machined with slots supporting radially-movable vane elements, mounted within a cam member and manifold having fluid inlet and outlet ports in the cam surface through which the fluid is fed to the low pressure inlet areas or buckets of the rotor surface for rotation, compression and discharge from the high pressure outlet areas or buckets of the rotor surface as pressurized fluid.
Vane pumps that are required to operate at high speeds and pressures preferably employ hydrostatically (pressure balanced) vanes for minimizing frictional wear. Such pumps may also include rounded vane tips to reduce vane-to-cam surface stresses. Examples of vane pumps having pressure-balanced vanes which are also adapted to provide undervane pumping, may be found in the aforementioned copending application and in U.S. Pat. Nos. 3,711,227 and 4,354,809. The latter patent discloses a vane pump incorporating undervane pumping wherein the vanes are hydraulically balanced in not only the inlet and discharge areas but also in the seal arcs whereby the resultant pressure forces on a vane cannot displace it from engagement with a seal arc.
Variable displacement vane pumps contain a swing cam element which is adjustable or pivotable, relative to the rotor element, in order to change the relative volumes of the inlet and outlet or discharge buckets and thereby vary the displacement capacity of the pump.
In conventional single acting vane pumps the rotor is splined upon and driven by a central drive shaft having small diameter journal ends which are not strong enough to withstand the opposed inlet and outlet hydraulic pressure forces generated during normal operation. This problem is overcome by forming such pumps as double-acting pumps having opposed inlet arcs and opposed outlet or discharge arcs which balance the forces exerted upon the journal ends, as disclosed by the prior art such as U.S. Pat. Nos. 4,354,809 and 4,529,361, for example.
Among the disadvantages of the latter known vane pumps is the necessity of two inlet arcs and two discharge arcs as compared to single acting pumps which have a single inlet arc and a single outlet arc. The shorter inlet arcs of dual-acting pumps requires that the vanes be pressure-loaded in the area of the inlet arc in order to cause the vane tip to track or maintain continuous contact with the cam surface. This results in higher vane-to-cam stresses and eliminates use of undervane pumping. The dual pump arcs also introduce leakage areas, which require side plates or end sealing plates to seal the ends of the rotor and the cam faces for the purpose of containing the pressurized fluid and avoid the creation of a high pressure gradient along the entire length of the rotor element and its journal ends. The present invention is concerned with improvements in such side plates or end sealing plates to produce vane pumps having improved efficiency and performance while reducing pressure loads exerted against the rotor in the pressure discharge direction.